Before Umar Farouk Abdulmutallab tried to detonate explosives hidden in his underwear on board a Northwest Airlines flight, he visited Islamic chat rooms where he wrote intimately about his struggle between liberalism and extremism.

And before Dzhokhar and Tamerlan Tsarnaev allegedly placed bombs on Boston's Boylston Street, Tamerlan watched YouTube videos of extremist Muslim preachers, and possibly plucked the recipe for a pressure-cooker bomb right off a virtual shelf.

The Internet has become crucial in the evolution of modern terrorism, experts say. Terrorism today is largely leaderless, executed by an informal network of disaffected misfits. Cloaked with a veil of anonymity, the discontent radicalize with the help of the Web's reassuring echo-chamber — hatred is stoked and jihad justified.

The Boston Marathon bombing appears to be a pernicious reminder that the Internet has extended the battlefield in the war on terror.

Terrorism expert Mitch Silber said that beginning in 2007, signs began to emerge that radicalization on the Internet was happening in the U.S. Silber co-authored the New York Police Department report, "Radicalization in the West: The Homegrown Threat."

Umar Farouk Abdulmutallab, shown in this Dec. 28, 2009, booking photograph, pleaded guilty to charges connected to his attempt to blow up a U.S. bound airliner using plastic explosives hidden in his underwear.(Photo: AFP/Getty Images)

"There was no outside ideological sanctuary for the Fort Dix 6 or the Times Square bomber," he said. "They learned about radical ideology online, and they self-propelled to launch terrorist plots."

Since then, Silber said, there have been half a dozen plots in which individuals were radicalized with the help of the Internet, which has become a vital accelerant for radical ideas. In the 2007 Fort Dix plot, six radical Islamist men conspired to attack U.S. military personnel in New Jersey. In the 2010 Times Square plot, Faisal Shahzad tried to detonate an explosives-filled SUV near the entrance to Manhattan's Minskoff Theatre.

The Tsarnaev brothers' activity on the Internet suggests they learned about radical ideology online, and had an interest in extreme Islamist figures years before the Boston attack.

U.S. officials said Tamerlan Tsarnaev frequently looked at extremist sites, including Inspire magazine, an English-language online publication produced by al-Qaeda's Yemen affiliate. Instructions on how to make bombs out of pressure cookers similar to those used in the Boston explosions were published in the magazine.

In August 2012, Tamerlan Tsarnaev created a YouTube channel with links to a number of videos with ties to radical Islamist causes. One features the firebrand Australian Muslim cleric Feiz Mohammed, who has called for Muslim youth to become holy warriors.

The SITE Intelligence group, which monitors Islamist websites, said Dzhokhar Tsarnaev followed someone on Twitter with the account "Al_firdausiA," which translates to "the highest level of Paradise, Allah willing." Among that user's tweets was a message encouraging readers to listen to an audio series by al-Awlaki.

While hard truths on the brothers' radicalization remain elusive, Dzhokhar Tsarnaev told law enforcement officials the brothers acted on their own.

Rabbi Abraham Cooper, who supervises the Simon Wiesenthal Center's Digital Terrorism and Hate Project, said almost all recent terrorist attacks on American soil have been "lone wolf" — individuals who self-radicalized, often with the help of the Internet, and either plotted or committed violence absent a direct command.

The Internet, Cooper said, offers a space where malcontents can find others who share their radical beliefs.

Army psychiatrist Nidal Hasan is scheduled to go on trial later this month on charges related to the 2009 Fort Hood shooting rampage that left 13 dead.(Photo: AP)

"There is a sense of empowerment and encouragement there that was completely non-existent before the Internet," Cooper said.

What many people may not know about the history of hate online, Cooper said, is the first groups to pitch their tent were Neo-Nazi, not Islamic-fundamentalist.

"For the first time, they didn't have to go through a producer of a show to get on, they didn't have to pass muster with a letter to the editor. They could do what they wanted and get to young people directly," Cooper said.

Islamic extremists took a look at tactics being used by these groups, who were anti-Muslim, and co-opted them, he said. Al-Qaeda now uses the Internet to target young Muslims all over the Western world.

The question becomes how to combat such an amorphous threat.

Cooper said there needs to be an organized "plan where we maximize freedom of speech but do a better job of interdicting efforts to promote and execute terrorism." Internet giants, governments from different democracies and free speech advocates need to sit down around a table together.

"The logical next step is not handing over privacy to government," Cooper said. "It's to work together to degrade the subculture of digital terrorism online."

In this May 2, 2010, image taken from video, a police officer approaches a vehicle containing a car bomb. He is reaching down to lift one of the red canisters on the roadway in New York City's Times Square. Faisal Shahzad was convicted in the case.(Photo: AP)